This invention relates to the use of indirect calorimetry within health management, in particular for use with ventilators.
In U.S. Pat. No. 5,989,188 Birkhoelzer et al. describe the use of an indirect calorimeter for determining the energy balance of a person. However, there is no description of how this would be achieved for a patient on a ventilator.
In U.S. Pat. No. 5,705,735, Acorn describes a method of determining nutritional requirements for a patient using an indirect calorimeter. However, the described system uses a pressure differential sensor to determine gas flow. The presence of a restriction in a flow tube can cause problems in medical applications. This system uses gas sampling for respiratory analysis, whereas the Applicant""s invention uses analysis of gases in the flow path, providing an effectively instantaneous analysis of gas composition.
In U.S. Pat. No. 5,647,370 Harnoncourt describes an ultrasonic spirometer. In this application, the transducers are at an oblique angle to the flow tube axis. In U.S. Pat. No. 5,645,071 Harnoncourt et al. describe a method for determining the molar mass of a gas mixture using an ultrasonic method. In U.S. Pat. No. 5,503,151, Harnoncourt et al. describe the use of ultrasonic transducers in analyzing respiratory gases. The use of these spirometers in a mechanical ventilator system is not described.
In U.S. Pat. No. 5,179,958, Mault describes an indirect calorimeter from which the respiratory quotient and resting metabolic rate can be determined. However, this device is not optimized for use with an intubated patient. The use of a carbon dioxide scrubber adds weight and volume to a respiratory analyzer.
In U.S. Pat. No. 5,285,794, Lynch describes a respiratory gas monitor; however this device uses a gas mixing chamber and does not provide real time measurements of flow rates and gas component concentrations.
Other patents describing the use of oxygen and carbon dioxide sensors for metabolic monitoring include U.S. Pat. No. 5,072,737 to Goulding; U.S. Pat. No. 5,069,220 to Casparie et al.; U.S. Pat. No. 5,060,656 to Howard; U.S. Pat. No. 4,856,531 to Merilainen; U.S. Pat. Nos. 4,619,269 and 4,572,208 both to Cutler; and U.S. Pat. No. 4,233,842 to Raemer et al.
U.S. Pat. Nos. 4,917,108; 5,038,792; 5,178,155; 5,179,958; and 5,836,300, all to Mault, a co-inventor of the present application, are incorporated herein by reference. These patents disclose systems for measuring metabolism and related respiratory parameters through indirect calorimetry. These instruments generally employ flow meters which pass both the inhalations and the exhalations of a user breathing through the instrument and integrate the resulting instantaneous flow signals to determine total full flow volumes. In some embodiments, the exhaled gases generated by the user are passed through a carbon dioxide scrubber before passing through the flow meter so that the differences between the inhaled and exhaled volumes is essentially a measurement of the oxygen consumed by the lungs. In other embodiments, the concentration of carbon dioxide exhaled by the user is determined by passing the exhaled volume through a capnometer and integrating that signal with the exhaled flow volume. The oxygen consumption can then be calculated as the difference between the inhaled and exhaled oxygen volumes, corrected to standard conditions.
Recently, James R. Mault, M. D. and others invented an improved indirect calorimeter, more fully described in U.S. application Ser. No. 09/630,398, the contents of which are incorporated herein by reference. The improved calorimeter comprises an ultrasonic detection apparatus combined with a fluorescence oxygen sensor. This improved calorimeter can be adapted for use with an intubated patient, or other patient connected in some manner to a mechanical ventilator or respirator.
The oxygen consumption of a person is related to their resting metabolic rate (RMR). This can increase up to several hundred percent in certain trauma victims, such as burn patients. In addition, the nutritional requirements of a person are also determined by their metabolic rate. An enhanced RMR can lead to muscle wasting of a patient, as muscle burning proceeds in order to supply the person with the required additional energy. Hence, for optimized recovery of a patient, it would be valuable to know their nutritional requirements.
In addition, the correct ventilation of a patient requires knowledge of carbon dioxide and oxygen levels in the blood. The carbon dioxide and oxygen levels in arterial blood can be determined using the end tidal gas component concentrations of exhaled breath.
The present invention provides an improved respiratory analyzer for use in a ventilator system, or other system to assist with breathing. An improved ventilator system for a patient comprises a ventilator unit providing respiratory gases, a tube (or line or conduit) for conveying respiratory gases to the patient, a flow module holder located within the tube (such as a slot, holder, clip, or the like); a flow module being be placed in the holder so that respiratory gases pass through a flow path of the flow module; and an electronics module, connected to the flow module and containing an electronic circuit having processor, designed to calculate a flow rate for respiratory gases flowing through the flow path. In a preferred embodiment, oxygen consumption volumes and metabolic rates are calculated by the electronics module.
It is an object of the present invention to provide an improved system by which the metabolic rate of an intubated patient can be determined.
It is a further object of the present invention to provide a system for improved respiratory control of a patient on a ventilator.
It is a object of the present invention to provide improved respiratory analysis for a patient on a ventilator or other means of respiratory assistance.